1. Field of the Invention
The present invention relates to a method of forming a thin semiconductor film and, more particularly, to a method suitable for forming a polysilicon film for a thin film transistor (TFT) on an insulating substrate.
2. Description of the Prior Art
Conventionally, a polysilicon film for a polysilicon thin film transistor (to be referred to as a polysilicon TFT hereinafter) is formed by the following method. A polysilicon film is formed on a substrate such as a quartz substrate by a low-pressure chemical vapor deposition method (LPCVD method) or an atmospheric pressure chemical vapor deposition method (APCVD method). Ions such as Si.sup.+ are implanted in the polysilicon film to convert the polysilicon film to an amorphous film, and annealing or thermal oxidation is performed to crystallize the amorphous film. With this method, crystal grains of the resultant polysilicon film are large. However, a TFT using this polysilicon film has a maximum electron mobility .mu. of about 100 cm.sup.2 /V.multidot.sec, which is insufficient for an application of silicon on insulator (SOI) (three-dimensional IC). When the thickness of a polysilicon film formed by the CVD method is less than 1,000 .ANG., the crystal grains are small and the trap density is large. Therefore, this polysilicon film has small electron mobility .mu. and lifetime .tau.. Furthermore, the polysilicon film has an electric conductivity .sigma..noteq..sigma..sub.0 exp(-E.sub. a /kT) (where E.sub.a is activation energy and T is absolute temperature) at room temperature and does not show an activation type conduction. The electric conductivity follows the rule of variable range hopping given by: EQU .sigma.=.sigma..sub.0 exp(-AT.sup.-1/4)
As a result, the above-mentioned polysilicon film has poor electrical characteristics.
Laser annealing can also be used to form a polysilicon film. With this method, an amorphous silicon film is formed by deposition on a substrate and is irradiated with a laser beam to grow crystal grains. A polysilicon film formed by this method, however, has poor electrical characteristics. For example, when a film of this type is used to fabricate a TFT, leakage current is large. In addition, uniformity is difficult to obtain when forming a large polysilicon film with this method.
In order to achieve an SOI application with a higher electron mobility .mu., crystal grains of the polysilicon film must be enlarged, and their orientation must be improved. Furthermore, in order to simplify device design, the size and orientation of the crystal grains should be easily controllable, and at the same time planar uniformity of the film must be achieved. In spite of various attempts using a laser or other methods, polysilicon films with a sufficiently large crystal grain size and a good crystal grain orientation cannot be uniformly formed at present.
A prior-art TFT reference is exemplified in the 45th Lecture Articles of the Japan Society of Applied Physics (1984), Nos. 14p-A-4 to 14p-A-6, PP. 407-408. This reference describes an improvement in a polysilicon TFT having transistor characteristics improved by an ultra-thin polysilicon film, improvements in a solid-phase crystal grain growth effect and conduction characteristics of the ultra-thin polysilicon film obtained by thermal oxidation, and an improvement in transistor characteristics obtained by annealing a structure in a hydrogen atmosphere at a temperature of 400.degree. C. after an Si.sub.3 N.sub.4 film is formed by a plasma CVD method on the ultra-thin polysilicon TFT to obtain the structure.